This patent application claims priority from a Japanese patent application No. 2001-103755 filed on Apr. 2, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a protection device. More particularly, the present invention relates to a protection device capable of preventing a problem that may be caused by connecting a DC power supply to a load erroneously with a reversed polarity.
2. Description of the Related Art
In an electrical apparatus, a load is made to operate in a forward connection state, i.e., a forward polarity connection, in which a positive input terminal of the load connects to an anode of a DC power supply while a negative input terminal connects to a cathode thereof. For example, a transformer is used in a vehicular apparatus as an example of the electrical apparatus. A voltage of an on-vehicle battery (an example of the DC power supply) that is connected to the primary side of the transformer by forward connection is changed to a higher voltage on the secondary side, thereby turning on a vehicular headlight.
On the other hand, the DC power supply and the load may be connected by a reversed connection, i.e., a reverse polarity connection, in which the positive input terminal of the load is connected to the cathode of the DC power supply and the negative input terminal of the load is connected to the anode of the DC power supply. This is, for example, a case where the on-vehicle battery is connected by the reversed polarity connection. In such a case, there would cause serious problems in, for example, breakdown of a circuit and meltdown of a fuse in the load.
Thus, in the conventional electrical apparatus, a protection device is arranged on an electric current path between the DC power supply and the load. The protection device has a switching portion that is turned on in the case of forward connection and turned off in the case of reversed connection, thereby preventing the aforementioned problems when the load is connected to the DC power supply by reversed connection, as disclosed, for example, in Published Japanese Patent Application Laid-Open No. 10-315849.
The protection device disclosed in Japanese Patent Application Laid-Open No. 10-315849 uses an FET or Field Effect Transistor as the switching portion and applies a control voltage to a gate of the FET from the anode of the DC power supply via a resistor.
When a large voltage such as a surge voltage is applied to the anode terminal of the DC power supply, this surge voltage may exceed the withstand voltage of the gate so as to damage the FET. Thus, the damage of the FET is prevented by means of a protection circuit including a capacitor or a Zener diode. For example, the protection device disclosed in Japanese Patent Application Laid-Open No. 10-315849 has a protection circuit including a capacitor.
However, in the conventional protection device, a large number of parts or components are newly required in order to supply the control voltage for controlling on/off of the FET to the gate terminal of the FET and the like and, therefore, would raise the manufacturing cost for the electric apparatus.
Therefore, it is an object of the present invention to provide a protection device, which is capable of overcoming the above drawbacks accompanying the conventional device. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the present invention.
According to the first aspect of the present invention, a protection device arranged on an electric current path between a load unit including a stabilized power supply unit and a DC power supply, comprises: a switching unit including an FET having a source terminal and a drain terminal arranged on an electric current path between the DC power supply and the load unit; and an input end to which a voltage generated by the stabilized power supply circuit in the load unit as a control voltage, the control voltage being capable of turning on the FET in a case of forward connection in which a positive input terminal of the load unit is connected to an anode of the DC power supply and a negative input terminal of the load unit is connected to a cathode of the DC power supply, and turning off the FET in a case of reversed connection in which the positive input terminal of the load unit is connected to the cathode of the DC power supply while the negative input terminal is connected to the anode thereof, wherein the control voltage is applied to a gate terminal of the FET via the input end.
The switching unit may be arranged at one of the positive input terminal and negative input terminal, the one terminal being a reference potential of the load unit.
The FET may be an n-channel type MOS-FET; the n-channel type MOS-FET may be arranged on the electric current path at one of the positive input terminal and the negative input terminal, that is a reference potential of the load unit; the drain terminal of the n-channel MOS-FET may be arranged at a polarity side of the DC power supply, the polarity side being the referential potential of the load unit, and the source terminal is arranged on the load unit side; and a parasitic diode may be formed between the source terminal and the drain terminal of the n-channel type MOS-FET in such a manner that a direction from the source terminal to the drain terminal is a forward direction.
The stabilized power supply circuit may be a series regulator operable to change an input DC voltage to a stabilized output DC voltage lower than the input DC voltage and to output the stabilized output DC voltage, and the output DC voltage of the series regulator may be applied to the input end as the control voltage.
The stabilized power supply circuit maybe a series regulator operable to change an input DC voltage to a stabilized output DC voltage and to output the stabilized output DC voltage; a resistor may be arranged between the input end and the gate terminal of the FET; and the output DC voltage of the series regulator may be applied to the input end as the control voltage.
The stabilized power supply circuit may be a switching regulator operable to change an input DC voltage to a stabilized output voltage to output the stabilized output voltage; the load unit further may include a sub-DC power supply unit operable to generate a DC voltage having a different magnitude from the output DC voltage in accordance with the output DC voltage of the switching regulator; the input end and the gate terminal of the FET may be directly connected to each other, and the DC voltage generated by the sub-DC power supply may be applied to the input end as the control voltage.
The stabilized power supply circuit may be a switching regulator operable to change an input DC voltage to a stabilized output DC voltage and to output the stabilized output DC voltage; the load unit further may include a sub-DC power supply operable to generate a DC voltage having a different magnitude from the output DC voltage in accordance with the output DC voltage of the switching regulator,; a resistor may be arranged between the input end and the gate terminal of the FET; and the DC voltage generated by the sub-DC power supply may be applied to the input end as the control voltage.
The summary of the invention does not necessarily describe all necessary features of the present invention. The present invention may also be a sub-combination of the features described above. The above and other features and advantages of the present invention will become more apparent from the following description of the embodiments taken in conjunction with the accompanying drawings.